We study experimentally the propagation of an in-plane crack through a
transparent Plexiglas block. The toughness is controlled artificially
and fluctuates spatially like uncorrelated random noise. The system i
s loaded by an imposed displacement and cracks in mode I at low speed
(10(-7)-5 x 10(-5) m/s). The crack front is observed optically with a
microscope and a high resolution digital camera. During the propagatio
n, the front is pinned and becomes rough. Roughness of the crack front
is analyzed in terms of self-affinity. The roughness exponent is show
n to be 0.55 +/- 0.05 in a static regime. No evolution of the roughnes
s exponent is observed during the propagation even if the crack speed
changes.